desktop_util: Add OS X version of open_in_file_manager()

According to 8680:

> “The open command opens a file (or a directory or URL), just as
> if you had double-clicked the file's icon. If no application name is
> specified, the default application as determined via LaunchServices is
> used to open the specified files.”
> (— `man open`)

So we can use the exact same code as with X11, only replacing the
launcher binary xdg-open with open. Notably, open does something
completely different on my Linux system.

This code is currently untested since I do not have an OS X machine of
my own.